The present invention relates to a preamplifier, and more particularly a preamplifier for use in an optical signal receiver.
An example of a conventional preamplifier for use in an optical signal receiver is shown in FIG. 1. In this FIG. 1, received light is converted into electric current by means of a photodiode. A preamplifier has a function of converting the current having been converted from the optical light into a voltage signal to output.
Here, there lies a predetermined relation between a gain A0 of the preamplifier and a signal bandwidth (fxe2x88x923dB). Namely, the signal bandwidth (fxe2x88x923dB) is determined by; total input capacity (Cin) consisting of capacity between the photodiode terminals, parasitic capacity caused by mounting, etc., and capacity of an FET constituting the preamplifier, etc., feedback resistance (Rf), and amplifier gain A0, in accordance with the following formula (1).                                                                         f                                                      -                    3                                    ⁢                  dB                                            =                              xe2x80x83                            ⁢                                                A                  0                                /                                  (                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    Rf                    *                    Cin                                    )                                                                                                        =                              xe2x80x83                            ⁢                              1                /                                  (                                      2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    Zin                    *                    Cin                                    )                                                                                        (        1        )            
(Here, Zin=Rf/A0 is an input impedance of the preamplifier.)
Now, because the amplifier itself normally has a bandwidth, it is necessary to design the signal bandwidth fxe2x88x923dB so that the signal bandwidth is sufficiently smaller than the amplifier bandwidth. However, in the optical signal receiver, there are cases of extremely small capacity between terminals, or on the contrary, extremely large capacity between terminals. Such cases depend on dispersion in the photodiode characteristic.
In the case when the capacity between terminals is extremely small, the total input capacity (Cin) becomes small. In this case the signal bandwidth fxe2x88x923dB is extended near to the amplifier bandwidth, as can be understood from formula (1). Thus oscillation or ringing is produced.
This oscillation or ringing condition will be explained referring to FIG. 2. FIG. 2A is shown by a scale width of 140 xcexcs. On the other hand, FIGS. 2B, 2C are shown by scale widths of 200 ns, in which a portion of the amplifier output period shown in FIG. 2A is enlarged. As shown in FIGS. 2B, 2C, a high-frequency oscillation signal is superposed onto a rectangular wave signal, which illustrates an oscillation or ringing condition.
To the contrary, in case the capacity between terminals of the photodiode is extremely large, the total input capacity (Cin) becomes large, producing an extremely reduced signal bandwidth fxe2x88x923dB. Therefore, as a portion of the preamplifier output period is enlarged in FIG. 3, there appears a condition that a rectangular wave signal component cannot be passed through. Such a condition may be understood from FIGS. 3B, 3C in which a portion period of FIG. 3A are enlarged with a scale identical to FIG. 2. The rectangular wave signal disappeared.
Meanwhile, when employing an APD (avalanche photodiode) as a photodiode, a photodiode APD has a characteristic of having small capacity when an input light signal is small, while having large capacity when an input light signal is large. There has been a known technology of preventing oscillation using this characteristic. That is, controlling a bias voltage of the photodiode APD by detecting preamplifier output amplitude which may vary in proportion to the input light signal of the photodiode APD (as having been disclosed in the official gazette of Japanese Unexamined Patent Publication No. Hei-7-304922, etc.)
The configuration block diagram in the above disclosure is shown in FIG. 4. In this FIG. 4, an output of the photodiode APD is input into preamplifier 2. An output of preamplifier 2 is led to AGC circuit 4, in which it is controlled so as to produce a constant gain.
A peak detection circuit 5 detects a peak value (amplitude) of an output of AGC circuit 4, to control a bias output of an APD bias control circuit 52 via a phase compensation circuit 51.
Now, a capacity value of the photodiode APD includes dispersion produced by a varying manufacturing condition, etc. In some cases, there is a photodiode APD which inherently has a capacity value causing oscillation or shortage of bandwidth. Therefore, when using a photodiode APD having such a capacity value in the configuration shown in FIG. 4, it is not possible to circumvent oscillation or shortage of bandwidth. The reason is, according to this configuration, that the amplitude detection for use in controlling bias voltage is carried out using a signal after passing through preamplifier 2 and AGC circuit 4.
Accordingly, it is an object of the present invention to provide a preamplifier which is capable of circumventing a problem of oscillation or shortage of bandwidth even in case an employed photodiode has a capacity value which inherently causes oscillation or shortage of bandwidth.
In order to attain the aforementioned object, according to the present invention, a band detection circuit is employed so as to detect a bandwidth of a preamplifier directly, thereby to control to obtain an appropriate preamplifier band using a band control signal being output from the band detection circuit.
As a preferred embodiment of the present invention, the preamplifier includes; a variable gain amplifier connected to a light receiving element for converting a received light signal to electric current; a variable feedback resistor connected to both an input and output of the variable gain amplifier; and a band detection circuit for detecting a signal band being amplified by the variable gain amplifier. Thus the band of the variable gain amplifier is controlled by a band control signal obtained from the band detection circuit.
As a preferred embodiment of a preamplifier according to the present invention, the aforementioned band detection circuit includes; a band detector; and a detection signal hold circuit for holding a detection signal obtained from the band detector. The aforementioned band detector includes; a level generation circuit for outputting two internally divided levels by resistance-dividing a bottom value and a peak value of a signal being input to the band detector; a first comparator for comparing an upper level side of the two internally divided levels being output from the level generation circuit with a signal being input to the band detector; a second comparator for comparing a lower level side of the two internally divided levels with a signal being input to the band detector; a first delay circuit for delaying the second comparator output for a predetermined time; and a comparison circuit for comparing the timing of an output of the first delay circuit with the timing of the first comparator output.
As another preferred embodiment of a preamplifier according to the present invention, the preamplifier further includes a bitrate detector, and an output terminal of the bitrate detector is connected to a bitrate signal input terminal of the band detection circuit.
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.